Determination of Multiple Metals in Cannabis Samples Using ICP-MS and ICP-OES

As an increasing number of U.S. states and countries enact laws to legalize the use of medicinal or recreational marijuana, there is a critical need to ensure product quality and safety. Similar to other consumer products such as foods and pharmaceuticals, cannabis testing needs to include the analysis of metals, some of which may be toxic if ingested or inhaled. Existing analytical methods used for plant-based samples can be applied to cannabis plant material and other cannabis-based products. A microwave acid digestion sample preparation procedure was verified using appropriate reference materials, and the digested cannabis samples were analyzed using both inductively coupled plasma–mass spectrometry (ICP-MS) and ICP–optical emission spectrometry (OES). The ICP-MS method used aerosol dilution to provide a highly robust plasma suitable for extended analysis of high-matrix sample digests. A fast ICP-OES method was also developed to analyze the same cannabis samples.

Cannabis-based products are available in a wide variety of formulations ranging from dry plant material, plant concentrates including waxes and distillates, and infused products such as foods and candies. Given the variety of sample matrices, existing sample preparation procedures developed for inductively coupled plasma–based techniques can be applied to cannabis products. For example, trace-element analysis of plant and nutritional supplement materials is a well-established application (1). Following acidic digestion to break down the primary components of the plant-based samples, inductively coupled plasma–mass spectrometry (ICP-MS) or ICP–optical emission spectrometry (OES) is often used for quantitative analysis because of the multielement capability, speed, and robustness of each technique. ICP-OES is suited to the analysis of mineral and micronutrients such as K, Ca, Mg, Cu, Fe, Mn, Zn, Cu, Mo, and Ni—vital elements required for plant growth. When the required analytes also include trace elements such as As, Se, Cd, Pb, and Hg, which may require lower detection limits, ICP-MS offers greater sensitivity, delivering detection limits and accurate analysis down to nanogram-per-liter (part-per-trillion) levels.

Most of the states that have legalized the use of marijuana for either medicinal or recreational use have enacted regulations for acceptable limits of toxic elements (Cd, Pb, As, and Hg) in cannabis and cannabinoid products (2). As shown in Table I (3), the limits can vary among states, and regulations governing the safety and quality of cannabis-based products are likely to evolve to include more elements. (See upper right for Table I; Table I: Example U.S. state regulations for heavy metals [3].)

Experimental

Preparation of Cannabis Samples

Two cannabis plant samples were analyzed in this study. Approximately 0.15 g of buds from each cannabis plant was weighed into a quartz vessel. Then 4 mL of nitric acid (HNO3) and 1 mL of hydrochloric acid (HCl) were added and the samples were microwave digested using a one-step program: ramp time of 20 min to a temp of 240 °C and hold time of 15 min. Hydrochloric acid was included to ensure the stability of Ag and Hg in solution.

The digested samples were diluted using a mix of 1% HNO3 and 0.5% HCl. National Institute of Standards and Technology (NIST) 1547 Peach Leaves and NIST 1573a Tomato Leaves standard reference materials (SRMs) were prepared using the same method to verify that the digestion was complete and confirm the quantitative recovery of the analytes.

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